1. Field of the Invention
The present invention relates to a mechanical automatic transmission apparatus for a vehicle which prohibits upshift and maintains a gear position, when running resistance at the time of start of the vehicle is large, to thereby prevent acceleration failure and downshift because of a drop in vehicle speed.
2. Description of the Related Art
In recent years, mechanical automatic transmissions in which actuators are added to a transmission gear mechanism and a clutch mechanism, which are similar to those used in manual-transmission vehicles, in order to enable automatic gear shift have been developed and put into practice, and applied mainly to large-sized vehicles such as trucks and buses.
In such a mechanical automatic transmission, in general, gear-shift control is performed on the basis of a gear-shift map as shown in FIG. 6. Specifically, upshift and downshift are performed on the basis of accelerator opening information and vehicle speed information.
For example, at the time of start, as shown in the gear-shift map of FIG. 6, the gear position is automatically shifted from 1st to 2nd when the vehicle speed reaches about 10-plus km/h, from 2nd to 3rd when the vehicle speed reaches about 20-plus km/h, and from 3rd to 4th when the vehicle speed reaches about 40 km/h. Each of the vehicle speeds for automatic upshift is shifted toward a higher speed as the accelerator opening increases.
However, when automatic shift control is performed on the basis of such a gear-shift map, a problem may arise when the vehicle starts with a large running resistance acting thereon; i.e., when the vehicle is on a steep uphill or carries a heavy load. That is, in the case where automatic shift control is performed by use of a gear-shift map, even when the vehicle is on a steep uphill or carries a heavy load, upshift control is automatically performed at a constant vehicle speed; e.g., upshift from 1st to 2nd is performed when the vehicle speed reaches 10-plus km/h, and upshift from 2nd to 3rd is performed when the vehicle speed reaches 20-plus km/h. However, in such a case, because of a large running resistance, drive force after upshift becomes insufficient, and the vehicle cannot be accelerated to a sufficient degree, so that a hunting phenomenon associated with gear position occurs in which the vehicle speed decreases and downshift is performed, and then upshift is again performed as a result of an increase in the vehicle speed.
A conceivable method for solving such a problem is estimating running resistance from acceleration and engine torque during traveling, and prohibiting upshift after start. According to this method, running resistance cannot be calculated unless the vehicle travels a certain distance or more after the start. Therefore, even in the case where upshift is desired to be prohibited immediately after the start, upshift may be performed because running resistance is calculated too late, and the above-described problem may occur.